Japanese Patent Application Laid-open No. 2000-190861 discloses an arrangement in which an electric power steering system is provided with motor drive disabling means for disabling the drive of a motor so that the motor, which assists a steering operation by a driver, is prohibited from generating a torque in a direction opposite to the direction of a steering torque applied to a steering wheel by the driver.
There is also a known arrangement in which an electronic control unit of an electric power steering system is provided with a main CPU (central processing unit) for carrying out main control of a motor and a sub CPU for monitoring the main CPU, and the sub CPU takes on the above-mentioned motor drive disabling function.
The main CPU has a function of calculating the actual current flowing through the motor in order to carry out current feedback control for making the actual current of the motor coincide with a target current, and similarly the sub CPU has a function of calculating the actual current of the motor in order to prevent a flow of current that has been prohibited by the motor drive disabling function. In both the main CPU and the sub CPU, the actual current of the motor is calculated based on a difference in voltage between opposite ends of a shunt resistor connected to an H-bridge circuit for driving the motor. However, since a predetermined offset voltage is detected due to variation in the motor current detection circuit and change in temperature even when no current is passing through the shunt resistor, a current value is calibrated with this offset voltage to be determined as the actual current of the motor.
However, if the timing with which the main CPU calculates the offset voltage and the timing with which the sub CPU calculates the offset voltage differ from each other, there is a possibility that different offset voltages might be obtained by the main CPU and the sub CPU due to variation in the power source voltage caused by the engine starting, etc., and in order to prevent this from happening it is necessary to provide a communication function between the main CPU and the sub CPU, and to synchronize, using this communication function, the timing of the offset voltage calculations of the main CPU and the sub CPU.
When a communication function between the main CPU and the sub CPU is provided in this way, it is necessary to diagnose whether or not this communication function is working normally, and furthermore, after the diagnosis it is necessary to carry out communication between the main CPU and the sub CPU in order to synchronize the timing with which the offset voltage is calculated, resulting in a problem that it takes time for the electric power steering to function normally after the engine is started.